I am currently studying Practical Electronics for Inventors, Fourth Edition, by Scherz and Monk. Chapter 2.5 Resistance, Resistivity, and Conductivity presents makes the following claim when discussing the concept of resistance:

Ohm's law can be applied only to ohmic materials -- materials whose resistance remains constant over a range of voltages. Nonohmic materials, on the other hand, do not follow this pattern; they do not obey Ohm's law. For example, a diode is a device that allows current to pass easily when the voltage is positive, but prevents current flow (creates a high resistance) when the voltage is negative.

I'm wondering what, if any, relationship there is between this explanation of diodes and the laser diode? The explanation provided is obviously very basic, but how does it, if at all, contribute to the functioning of laser diodes?

I would greatly appreciate it if people would please take the time to clarify this.


2 Answers 2


I'm wondering what, if any, relationship there is between this explanation of diodes and the laser diode?

If you read the Wikipedia page that you linked to, you'll see that a "laser diode is like an LED".

A semiconductor junction diode works because the arrangement of the P-type materials and N-type materials allow current to flow in just one direction.

An LED (which is a kind of semiconductor junction diode) emits light because of some special quantum mechanical properties of the materials used such that when it conducts, instead of the energy dissipated by the diode drop times current generating heat, at least some of it generates photons.

A laser diode (which is a kind of LED) undergoes Light Amplification by Stimulated Emission of Radiation (LASER), because as it conducts in leaves a bunch of electrons in an excited state (called a population inversion), and when a photon smacks into such an electron the electron falls to its ground state and generates another photon in the same direction and phase as the first -- which is what the laser effect is all about.

You could possibly use a laser diode as an "ordinary" diode, but it would be unhappy. They're quite specialized devices -- but under the hood, there's a PN junction, and many of the same quantum-mechanical processes that make a plain old 1N4148 conduct current in one direction but not the other are present in a laser diode.

  • \$\begingroup\$ Thanks for the answer. So is there a clear, fundamental relationship between the diode and the laser diode (LED)? It's difficult for me to discern a clear relationship from what you have written. \$\endgroup\$ Jan 25, 2020 at 19:32
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    \$\begingroup\$ I've edited slightly, but a laser diode is a kind of LED (i.e., the set of all laser diodes is a full subset of all LEDs), and an LED is a kind of semiconductor junction diode (i.e., the set of all LEDs is a full subset of all semiconductor junction diodes). So, a laser diode is a semiconductor junction diode. \$\endgroup\$
    – TimWescott
    Jan 25, 2020 at 19:46

Bespoke Current-Voltage relationships do not provide explanations of how light emitting diodes work. It is characterization of the input / output relationship: not the lasing mechanism.

As an analogy: the relationship between my Toyota Camry's velocity (measurable output) and accelerator position (controllable input) does not explain the mechanism (Pressure-volume work) of why the vehicle moves forward.

Resistors are linear models of current produced when a voltage (battery) is applied. In contrast: Diodes are NONLINEAR voltage-current devices. This graph shows the output current as a function of the voltage across the diode (see bottom right of diagram).

enter image description here

Said explanations and relationship only address voltage-current relationships and not luminosity (light emitting capability). The voltage-current relationships are important (useful) in selecting resistors to limit reasonable currents through LEDs and not damage the LED.

  • \$\begingroup\$ Thanks for the answer. Your link fails to open. Also, can you please describe what the image is showing? \$\endgroup\$ Jan 25, 2020 at 18:23
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    \$\begingroup\$ Link now works. Sparkfun provides excellent introductory device operations explanations. Bespoke Current-Voltage relationships do not provide explanations of how light emitting diodes work \$\endgroup\$
    – 411
    Jan 25, 2020 at 18:36
  • \$\begingroup\$ Mmm, the website descriptions for forward-bias, reverse bias, and breakdown were very informative. Thanks! \$\endgroup\$ Jan 25, 2020 at 19:24
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    \$\begingroup\$ @ThePointer I recommend reviewing the other tutorials in Sparkfun because they provide key practical insights that can help make sense of the very abstract theory. \$\endgroup\$
    – 411
    Jan 25, 2020 at 19:50

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